Currently using the Kindle for iOS app because I can run it on my iPad and my iPhone and sync between them. It is also somewhat important to me to be able to sync with the Kindle for Mac/PC apps. I don't like the Kindle apps due to their lackluster format options, but they are the only ones that supply all my needs. I like Marvin on the iPad, but it still isn't available on the iPhone. Even when the long awaited iPhone app is released, I wouldn't be able to sync wirelessly between computer and iPhone and iPad, at least not without some ridiculous and kluge process. So for now I will stick with Kindle apps.